Various communication systems are known in the art. Pursuant to many such systems, an information signal is modulated on to a carrier signal and transmitted from a first location to a second location. At the second location, the information signal is demodulated and recovered.
Typically, the communication path used by such a system has various limitations, such as bandwidth. As a result, there are upper practical limitations that restrict the quantity of information that can be supported by the communication path over a given period of time. Various modulation techniques have been proposed that effectively increase the information handling capacity of the communication path as measured against other modulation techniques. Sixteen-point quadrature amplitude modulation (QAM) provides a constellation of modulation values (distinguished from one another by each having a different combination of phase and amplitude) wherein each constellation point represents a plurality of information bits.
FIG. 1 shows a constellation for a 16 QAM communication system that is a map of 16 points on the complex plane defined by a horizontal axis representing the real portions, and a vertical axis representing imaginary portions, of a complex number. Transmitted QAM information symbols on a communication channel (and the pilot and sync symbols as well) are discrete packets of a carrier signal modulated to convey information using both the amplitude and phase-angle displacement of the carrier from some reference. QAM information symbols are represented on the constellation of FIG. 1 as complex quantities represented as vectors having both magnitude (represented as length or distance from origin) and phase angles (which angles are measured with respect to one of the axes). In a 16-QAM system, having sixteen (16) different magnitude and phase angle combinations that correspond to sixteen (16) different possible bit patterns of four binary digits (which bits are from a serial stream of bits from an information source), each of the sixteen (16) points on the constellation is identified as representing one combination of four bits.
A vector (10) (expressed in rectangular coordinates as 3+3j and having a length=(3.sup.2 +3.sup.2).sup.1/2 and a phase angle (12) equal to the arctan of 3/3 or forty five degrees with respect to the real axis), points to the point {3,3j} on the constellation, which point is shown in FIG. 1 as representing the series of four binary digits, (0,0,0,0). A second QAM symbol (14) points to yet another point (1, -1j) in this constellation and represents four other digital symbols.
From the foregoing, it can be seen that eight bits of information can be represented by two, 16-QAM symbols. When a digital stream is directly converted to 16 QAM, four-bit blocks of the data are mapped to the various vectors that correspond to the bit pattern embodied in the four bits. When the QAM symbols that represent the digital information are transmitted, the symbols are transmitted with amplitudes and phase angles that correspond to the magnitudes and phase angles of the vectors used to represent the various patterns, such as those shown for the vectors 10 and 14 depicted in FIG. 1.
In addition to modulating the information signal in a typical manner described generally above, it might also be desirable to disguise (encrypt) the information signal prior to transmission, as well as to provide some form of error detection and/or correction. The drawbacks to doing so is that both encryption and error correction schemes generally add significant undesirable processing delay to the information transfer process, and particularly to voice transmitted information.
Regarding encryption, because the radio signals are sent over public space, there is always the possibility of someone capturing the signal and copying it for improper use. One approach is to avoid sending any critical, confidential or restricted information of the information stream or signal packet over the air. The preferred approach, however, is to apply some sort of encryption algorithm to the data portion of the information stream and then having the proper receivers possess a decryption key which would make possible extracting the true contents of the data from the encrypted packet.
Typically, encryption schemes are implemented (architecturally) as shown in flow diagram of FIG. 2. During information processing, the information stream (e.g., a voice signal, packet data, etc.) is formatted (20) into a form that can be handled in the wireless environment. Thereafter, if encryption is employed, the formatted output is encrypted or disguised (30) to provide the necessary protection for safely transmitting the formatted signal over the air. The next stage typically involves encoding (40) the encrypted signal (digital bit stream) to provide a means for identifying and correcting errors. Typically, errors in radio-frequency (RF) communication systems are usually associated with interference that modifies the data streams while they are in the transmission medium (typically air). Since errors can cause changes or dropping of information bits and thus invalidate the received packets, error correction is quite critical, particularly in non-voice or packet data information transfers.
Once the system network has progressed through each of the formatting, encryption and encoding stages of information stream processing, the network forwards the encoded information to the modem layer of processing where it is modulated (50) (using for example a 16-QAM modulation technique)--and then transmitted over the air.
On the receiver end, the received packet is demodulated (60), decoded (70), decrypted (80), and deformatted in to a form recognizable by the receiving party (voice) or apparatus (packet data). The decoding and decryption schemes provided at the network layer by the network are based on the corresponding encoding and encryption schemes employed by a similar layer in the network. Similarly, the demodulation scheme employed at the modem (or physical) layer of the receiver is based on the corresponding modem layer modulation scheme at the transmitter. Encryption (decryption) and encoding (decoding) are therefore higher level mechanisms and network dependent, while modulation (demodulation) is system independent and typically a mechanism residing in the modem layer of the information transfer process.
It would be a great advantage in the art to be able to introduce encryption functionality to the information transfer process, but to do so in a less intrusive manner such as outside the higher (network) layer of processing which adds significant undesirable processing delay to information transfer.